The role of the power transmission utility industry in providing reliable service is under increasing pressure in today's wholesale power market, which must manage an increasing number of transmission modes, brought upon by deregulation. Power generation outages or transmission line faults several systems away can produce voltage disturbances throughout interconnected power systems. Transmission planning done for individual systems decades ago did not anticipate these changes, nor the higher power quality standards that would be required by today's critical manufacturing processes, such as for example, semiconductor and integrated circuit fabrication. From a utility company's perspective, the degree of reliability of power must be "good enough" for the general public, and added enhancements for a particular industry face difficulty obtaining regulatory approval if the costs are to be borne by other electric customers.
Deregulation of the Utility Industry and increased sharing of existing utility grid networks are expected to result in a further decline in the quality of electric power available for industrial consumers. In a deregulated environment utilities will begin to minimize investment and maintenance expenditures and therefore the grid infrastructure will become older and less reliable, thereby decreasing power quality. Momentary sags and power interruptions cause at least $26 billion in downtime in terms of productivity in the United States alone. Lost revenue due to power quality problems for a typical 200 millimeter wafer semiconductor manufacturing factory in the United States is estimated to be in the range of $20-$50 million per year per plant.
Exacerbating the problem are the variety of entities involved in supporting semiconductor manufacturing, none of whom take full responsibility for ownership of the power quality problem. The industry trend is toward higher performing equipment within plants which may typically lead to greater sensitivity to a voltage disturbance. Existing solutions for distributed power quality within a plant, such as a conventional uninterruptable power supplies (UPS), often create unacceptable harmonic distortions in power, thereby increasing instability and leaving gaps in protection that are discovered piecemeal as particular plant or grid operating scenarios are developed.
There is an increasing need for clean, uninterrupted electric power to be provided for today's power sensitive industrial processes. For example, the optical industry, hard disk production, textiles, paper mills, plastic foil production or other complex processes involving rotating machinery incur severe economic losses in terms of damaged product and down time when there is a power interruption or undervoltage condition on the utility grid. In particular, semiconductor manufacturing processes are especially sensitive to interruptions, undervoltage conditions or any discontinuity on the utility grid supplying power to the plant.
The increasing demand for semiconductor wafer manufacturing plants to provide smaller, faster integrated circuits with device dimensions which are approaching the wavelength of visible light has created an urgent need for clean, stable, uninterrupted electric power. As semiconductor wafer processing increasingly requires lithography at deep submicron dimensions, the complex series of lithographic process steps and positioning of wafers become extremely sensitive to even slight variations in power.
Miniaturization, which has been the driving force for achieving performance and cost improvements in very large scale integrated systems (VLSI), emphasizes more reliable VLSI devices as well as higher performance. The objective today in both high speed logic and fast memories is toward higher integration levels. Higher integration levels are seen as the key to obtaining higher device performance. At submicron dimensions, even slight variations in power or minor voltage discontinuities for as little as 50 milliseconds can result in losses of wafers containing integrated circuits worth millions of dollars. Refer to FIG. 7.
The sensitivity of modern VLSI technology to even slight variations in power can be seen from the following example. Major applications in MOS technology as well as increasing use of bipolar structures include polysilicon gate electrodes and interconnects. Poly layers in direct contact with the silicon substrate are used as diffusion sources and buried contacts. High performance devices are realized by means of the extremely high resistivity of lightly doped polysilicon. In device fabrication applications, poly structures must be exposed to an entire range of process technologies such as oxidation, diffusion and implantation. These processes are very sensitive to even slight voltage variations.
Further, VLSI structures and devices are inherently multi-layered with multiple interfaces whose properties may be crucial to the resulting device behavior. As dimensions shrink to 0.25 microns and below, even minor variations in power can detrimentally affect the extreme precision which must be adhered to when implementing VLSI fabrication processes. Processes such as reactive ion etching, plasma enhanced chemical vapor deposition (CVD), diffusion, and ion implantation are inherently electric powered based. Other methods can be used to shrink dimensions of integrated circuits even further, such as extreme ultraviolet lithography (EUV), x-ray lithography and electron beam lithography. Since the foregoing processes are arguably capable of shrinking dimensions smaller than 0.1 microns, such processes are extremely sensitive to undervoltage conditions such as voltage transients induced by lightning, interruptions or sags in voltage due to increased utility demand, or simply an inability to provide clean power due to varying industrial loads.
As lithographic processing becomes ever more complex, it becomes necessary to provide a stable, uninterrupted source of power to steer electron beams or conduct other lithographic processes with complete, invariant accuracy. Power discontinuities which may have been tolerated even a few years ago are now unacceptable due to the fact that the extremely small device dimensions now magnify any power deviation. Also, the more exacting semiconductor processing technologies are creating an increased power demand. Consequently, semiconductor processing plants are operating at higher electric power levels.
In order to solve the problems in meeting increased power demands and providing an uninterrupted source of clean power to a critical manufacturing process or the like, one conventional approach is the use of distributed power protection (uninterruptable power supply or UPS) at the equipment level. However, implementation of this solution has proven difficult and only partially effective for the following reasons.
It is difficult to identify the critical loads that require immediate protection since the priority of the loads may be changing in accordance with the specific semiconductor processing step being undertaken. Distributed power protection such as UPS at the equipment level also has proven impractical because it is difficult to segregate critical and non-critical loads within a plant. Also, conventional UPS suffers from a lack of industry standards. Consequently, the variety of UPS equipment suppliers has created significant compatibility problems. In addition, studies have found that the proliferation of distributed power protection at the equipment level creates significant problems in terms of internal harmonic pollution. That is, the numerous UPS or power surge protection devices can create unwanted harmonic effects throughout the power system at the equipment level, which further tends to destabilize power on an industrial plant's internal power grid.
Conventional solutions to the problem of electric power transmission stability also may include a superconducting magnetic energy storage (SMES) system such as exemplified in U.S. Pat. No. 4,695,932. A typical connection scheme of a conventional SMES system is shown in FIG. 2. A conventional power conditioning system such as SMES or similar method may not react swiftly enough when a voltage sag is detected. Also, conventional SMES systems, as will be explained infra, generally can not provide back up power to the load fast enough and without creating destabilizing transient voltages. As presently designed, a conventional SMES system also fails to provide a stable, interrupt-free source of power for high power, multiple load applications.
Conventional superconducting energy storage circuit solutions are pulsed in nature and thus require special attention in the area of AC losses and conductor stability under pulsed operation. AC losses during standby mode, due to the ripple introduced by power electronics, motivated prior SMES solutions to use two independent power supplies, one for charging a superconducting magnet, and one for discharging to the load. These power supplies, being rated independently for charging and discharging duty, in turn elicit the need for at least two switches that must be synchronized and activated simultaneously during voltage sags or interruptions. Such a conventional circuit topology leads to a slow response in providing back up power to a load. At high power levels, and using conventional circuit topology, it would not be possible for a conventional SMES to respond fast enough to protect the load.
It is desirable to connect the storage to the load and the grid through a single power converter and a single switch. Recent advances in power electronic devices allow for the construction and implementation of a power converter with reduced on-state losses.
A conventional SMES power converter uses a gate turn off device (GTO) which typically has a voltage drop of 3.5 volts when closed. When open, the stand-off rating is approximately 4,000 volts. The on state losses are determined by the current multiplied by the voltage. At 15,000 amperes, a conventional SMES incurs on-state losses in a range of about 10-100 kilowatts.
In contrast, an aspect of the present invention uses gate commutated thyristors (GCT) which experience on-state losses of only approximately 2.7 volts per switch. That is, a GCT has an on-state voltage drop of only approximately 2.7 volts as opposed to the usual 3.5 volts for a conventional GTO.
AC losses in a conventional SMES system therefore are a limiting design aspect which prevents a conventional superconducting energy storage circuit from supplying back up power, free of potentially damaging voltage transients, to a power sensitive load such as a semiconductor manufacturing plant. Unacceptable AC losses also act as a design limitation and prevent a conventional SMES system from providing back up power to a plurality of loads such as an industrial park.
In an attempt to solve the problem of unacceptable AC losses, U.S. Pat. No. 4,695,932 uses a separate AC/DC converter to trickle charge the superconducting magnet and teaches the use of a separate AC/DC converter to support power to the load and adds a capacitor between a chopper and AC/DC converter circuit. This added complexity requires at least two points of connection and alternate circuit paths between the utility grid source and the load. See FIG. 2. The additional complexity of the connection including the use of a separate AC/DC converters to charge the superconducting magnet as well as a separate AC/DC converter to support power to the load significantly slows down the response of the SMES to detection of a potentially damaging power sag. The added complexity also increases the occurrence of voltage transients and distortions upon connecting the superconducting magnet to the load. This conventional design precludes the use of this type of SEMS system for protecting a large industrial load, such as an entire semiconductor processing plant which is sensitive to voltage transients.
U.S. Pat. No. 5,329,222 is directed to a system for compensating for utility distribution line transients such as voltage sags. The system uses an energy storage system and inverter for generating a voltage which is injected in series with the distribution line voltage. The generated voltage does not provide full power to the line, but rather only compensates for differences from a desired utility reference voltage. A major disadvantage is that the system cannot restore an active power vector without storage. Such a conventional system is not capable of supporting a complete power outage on the grid, it is only effective during voltage sags.
Another disadvantage of this teaching is that it lacks enough storage capacity to compensate for a complete outage on the utility distribution line. Inherent design limitations imposed by the inverter and the inability to provide a complete disconnect from the distribution line further preclude the application of this type of back-up energy system for controlling a plurality of loads depending from a single utility substation.
A conventional shunt connected superconducting energy stabilizing system such as described in U.S. Pat. No. 5,514,915 has at least two points of connection and two circuit paths between a power source and a load. The superconducting magnet is fed by an AC/DC converter which is separately coupled to the power source through one of the circuit paths. During periods of voltage sag or a power outage, an isolation switch on another circuit path provides isolation of the load from the power source so that energy can be supplied to the load from the superconducting magnet through a DC/AC converter. This system uses separated converters on separate circuit paths for both energy directions from and to the magnet. Accordingly, it is very difficult to switch or to coordinate both circuit paths simultaneously in the event of a voltage sag or power outage, especially at higher loads. Also, such a system has a disadvantage of added complexity since special control means are required.
The teaching of U.S. Pat. No. 5,514,915 could not be used to interface between a utility substation and a multiple power sensitive high output loads such as a plurality of plants comprising an industrial park, for example. The separated converters on separate circuit paths and two points of connection to and from the load and the utility grid impose severe design limitations which preclude the adaptation of this system for high power applications. A major problem is the complexity involved in coordinating the opening and closing of high power switches. The switching timing becomes increasingly difficult to achieve at such high power applications. This application can not be used to quickly ramp-up to the desired load operating current without creating voltage transients induced by the connection to the utility source. This solution will only work for lower power applications.
U.S. Pat. No. 5,376,828 also discloses a conventional shunt connected SMES having two separate circuit paths between a utility grid and a load. Separate converters keep the energy storage system charged. As in U.S. Pat. No. 5,514,915 separate converters are necessary for keeping the energy storage component charged and for full power discharge to support the load. The additional components and separate circuit paths appear to be necessary to compensate for small perturbations in the grid without having to exercise the entire superconducting magnet or energy storage system. This would be necessary to avoid energy losses when exercising the full switching of the grid into the energy back-up and recovery system. Full switching of the grid into the energy storage system unnecessarily exercises the superconducting magnet and imposes the danger of overloading the superconducting magnet if the current from the grid is not carefully controlled.
In order to overcome problems in enhancing utility power standards as required by refinements in power sensitive manufacturing processes, what is needed is a system which can provide plant-wide protection between the utility grid and a power sensitive industrial application such as semiconductor manufacturing, optical processes, hard disk production, integrated circuit fabrication, or any complex manufacturing process involving rotating machinery.
In view of the critical importance for providing uninterrupted and smooth power to all phases of VLSI device manufacturing, what is also needed is an energy management system for detecting, within milliseconds, any type of potentially damaging power distortion appearing on a utility grid, and for substantially instantaneously disconnecting the load to be protected from the grid. At the same time, the energy management system must provide an uninterrupted source of clean power to the load with substantially instantaneous ramp up to the necessary power level and deliver that power without distortion to the load.
It also would be advantageous to provide a source of back up power with only one circuit connected between the utility source and the load and having only one point of connection/disconnection required between the utility source and the load. When a power transient is detected on the utility line such a system advantageously would be able to switch off from the utility grid and connect power immediately without creating voltage transients.
It also would be desirable to provide a system which eliminates the need for separate converters for keeping the energy storage system such as the superconducting magnet charged and ready for full power discharge to support the load. The use of a single DC/AC converter could be used to keep both the energy storage system charged during stand-by or discharge its energy during voltage sag protective operation. This advantageously would provide only one switch between the utility grid and the load and thus would allow faster ramp-up of power to the load without the voltage transients inherent in a separate connection to the utility power grid. Such a system also should be capable of providing continuous reactive power voltage control.
What is also needed is a SEMS with a capability of providing a variable impedance in order to create a virtual grid. This would enable a SEMS to increase voltage regulation to a load for perhaps up to ten percent, rather than the conventional five percent without exercising the superconducting magnet or other energy storage of the SEMS. While the system is connected and while the switch to the SEMS is closed and the load is connected to the grid, it would be advantageous if the power connection of the SEMS could be used to regulate the voltage on a continuous basis, without exercising the energy storage of the SEMS or the switch to the grid. This voltage regulation can be done within limits which depends on the reactive power rating of the converter and the short circuit power of the grid to which the load and SEMS are connected.
For example, if the load is connected to the grid with a short circuit power of 1,000 megavolt amperes (MVA) and the power control system (PCS) is rated at 50 MVAR, then a SEMS should be able to regulate power on a continuous basis to five percent without exercising the magnet. Accordingly, there is a need to increase the range of continuous voltage regulation without exercising the stored energy of the superconducting magnet.
The foregoing attempts to solve the problem of utility voltage instability delivered to a power sensitive load are restricted to relatively low power applications due to inherent design constraints on the SMES system which also limits the size of the superconducting energy storing magnets. None of the foregoing conventional SMES system applications would be capable of interfacing between the utility grid and a plurality of power sensitive customer loads, such as an industrial park or a plurality of semiconductor manufacturing plants.
Therefore, what is also needed is an energy management system for interfacing between a utility grid and one or more manufacturing plants, such as an industrial park, which accurately can predict the onset of a voltage disturbance, completely truncate that disturbance from a selected load and provide a substantially uninterrupted, stable source of power to the selected load for the duration of the interruption or until back-up power generation is brought on line. This advantageously would eliminate the present need for a multiplicity of power control circuitry located throughout a plant which itself creates harmonic interferences and power disturbances within a plant's own power grid.
Another shortcoming of a conventional SMES system as described above is the inability to provide substantially instantaneous ramp up of back-up power to the load in an invariant, expected manner such that the critical power characteristics of the supplied power conform to a predefined set of industrial power quality parameters.
Therefore, what is also needed is a predictable source of back-up power which can be provided substantially instantaneously when needed. It also would be desirable if the critical response time and voltage/current levels of the back-up power always conformed to an industrial power quality standard that governs the operational parameters of the load. For example, semiconductor manufacturing plants universally require operational characteristics within the parameters of a practical undervoltage operating limit. Examples of such practical undervoltage operating limits comprise the so-called CBEMA curve or the ITIC curve. A practical undervoltage limit provides a standard measure for available response time so that equipment is maintained within functional operating parameters. (See FIG. 7). What is needed is a back up energy management system which in every event could be relied upon to invariantly provide substantially instantaneous back up power with characteristics which conform to the parameters of a practical undervoltage limit and which could maintain equipment within expected operating parameters.
Another problem with conventional SMES technology is the production of an unwanted, potentially adverse magnetic field extending beyond the cryogenic enclosure. A strong magnetic field can have a serious effect upon persons wearing old style heart pacemakers, for example. Therefore, what is also needed is a superconducting energy management system which has a magnetic field at the facility fence line which is within an acceptable level of exposure to workers.